The present invention relates to a device for dewatering material suspensions. More particularly, the present invention relates to a device comprising two co-operating cylindrical rotary rolls, where at least one of the rolls is liquid permeable. The rolls form a gap between them, through which the material is intended to pass downwardly from above and simultaneously be dewatered.
The material suspension can, for example, be a pulp suspension, and the dewatering device can be a washing press or a dewatering press.
Dewatering devices known in the art are described herein where both rolls are liquid permeable. The rolls are formed with liquid permeable shell surfaces, which consist of a perforated metal sheet attached to a roll body. In order to achieve high capacity, the total open hole area must be substantial, and at the same time the holes must be small, in order to prevent fine material, for example small fibers, from following along with the liquid through the holes. In order to solve these problems, the rolls can be provided with a wire cloth on the outside of the perforated shell metal sheet. The wire cloth allows liquid flow along the shell surface, and usually is formed of plastic wires. This permits the perforations to be designed greater so as to yield a higher capacity without the risk of an increased passage of fine material.
The rolls are normally rotatably mounted on two parallel shafts. The material suspension is supplied to the device so that it is moved by rotation of the rolls downwardly from above through the nip, whereby compression and dewatering of the material to a desired dry matter content takes place.
Below the nip, the transfer of material from the rolls takes place by means of a doctor device and gravity. A doctor blade abuts the shell surface of each roll and extends along the entire length of the roll. The roll can have a length of up to 8 meters. The doctor is intended to ensure transfer of the material from the rolls, and thus mechanically scrapes off the material. When the rolls are covered with wire, the doctors are arranged at a small distance, usually from about 0.1 to 0.5 mm, from the roll so that the doctors do not damage the wire cloth.
A dewatering device of this kind is described, for example, in the Swedish patent specification No. 504,011.
When only one of the rolls is liquid permeable, it is designed as described above and, consequently, has one doctor device for transferring the material. The roll, which is not liquid permeable, is in this case primarily provided to render the formation of a nip possible.
During such dewatering problems arise with wear and deflection of the doctors. The requirements on the doctors are greater, and the doctors must abut the roll without the force between doctor and roll being so great that the doctor and roll are subjected to high mechanical stresses. In spite of highly accurately designed and rigid doctor devices, problems nevertheless arise with wear of both the doctors and the rolls. Due to such wear, the doctors must be exchanged often. They are also quite expensive with regard to their construction and manufacture. Wear of the rolls results in deterioration of their function and shortening of their life.
When the roll is covered with a wire, it is desired to maintain a space of tenths of a millimeter between the doctor and the wire. This is not made easier by the fact that the doctors get worn, and in certain cases problems arise because the wire is damaged by the doctors.
During dewatering with devices of these types, there also arise problems because of the re-wetting of the dewatered material . When the dewatered material leaves the nip, the compressing force decreases. Pressed-off liquid (filtrate) can then run back through the perforated metal sheet and re-wet the material, whereby the washing result and dry matter content are deteriorated.
Experiments have shown that the filtrate running back after the nip preferably deposits as a liquid film on the roll. The liquid film then follows along with the roll in its rotation. The doctor, which generally abuts the roll, scrapes off this film so that it follows along with the dewatered material and re-wets the same.
In accordance with the present invention, these and other difficulties have been overcome by the invention of apparatus for dewatering aqueous suspensions comprising a pair of rotatable rolls juxtaposed with each other to form a nip therebetween, whereby the aqueous suspension can pass downwardly through the nip and be dewatered by removing a filtrate from the aqueous suspension and the dewatered suspension can pass downwardly from the nip, at least one of the pair of rotatable rolls being liquid permeable whereby a film of the filtrate forms on the at least one of the pair of rotatable rolls below the nip, and at least one screen adjacent to the at least one of the pair of rotatable rolls disposed below the nip and having a first side facing the at least one of the pair of rotatable rolls and a second side facing away from the at least one of the pair of rotatable rolls, the screen including an upper end proximate to the at least one of the pair of rotatable rolls and separated from the at least one of the pair of rotatable rolls by a predetermined distance and extending along its length, the predetermined distance being a sufficient distance whereby the film of the filtrate passes between the at least one of the pair of rotatable rolls and the upper end of the screen on the first side of the screen and the dewatered suspension passes on the second side of the screen so that the filtrate is prevented from contacting the dewatered suspension below the nip. In a preferred embodiment, each of the pair of rotatable rolls is liquid permeable, and the apparatus includes a pair of the screens disposed below the nip adjacent to each of the pair of rotatable rolls. Preferably, the distance separating the pair of screens from each other increases in a direction away from the nip.
In accordance with one embodiment of the apparatus of the present invention, the predetermined distance is between 1 and 100 mm.
In accordance with another embodiment of the apparatus of the present invention, the distance between the at least one of the pair of rotatable rolls and the screen increases in a direction moving away from the nip.
In accordance with the present invention, a screen device is arranged so that the distance between the screen device and the roll is of a magnitude such that the liquid film on the roll passes between the screen device and the roll, thereafter without coming into contact with the dewatered material, runs down into a filtrate collecting means.
According to the present invention, re-wetting of the dewatered material is reduced considerably. Due to the fact that the doctor is removed, the mechanical effect between roll and doctor, and thus all of the costs in connection with the exchange of worn doctors, are thereby eliminated. The risk of damage of the wire cloth or roll is also eliminated.
In addition, the requirements for accuracy of the design of the screen device and its rigidity are much lower than in the case with a doctor device, because the screen device is located at a relatively great distance from the roll. The present invention, therefore, results in a reduction of the costs related to construction and manufacture compared to the use of the doctor device.
By thus utilizing apparatus according to the present invention, the dry matter content of pulp can be increased from 2.5 to 8% to 20 to 40%, preferably from 3 to 8% to 30 to 35%.